Switchable antenna for the VHF and UHF frequency bands

ABSTRACT

This invention concerns an antenna switchable between the VHF and UHF frequency bands. This antenna incorporates a capacitive element, disposed at a specified distance from a reflecting surface mounted on the body of the vehicle. For VHF, tuning is obtained by a fixed self-inductance in parallel and a variable self-inductance in series with the capacitive element. The latter comprises various sections which can be short-circuited by switches. For UHF, the variable self-inductance is totally short-circuited and the fixed self-inductance is disconnected by a switch; then, plates form, with the reflecting surface and the capacitative element, a &#34;manchette&#34; antenna. This switchable antenna can in particular be mounted on an airplane.

BACKGROUND OF THE INVENTION

The present invention relates to a switchable antenna for the VHF andUHF frequency bands, intended in particular for aircraft.

Numerous aircraft, and especially military jet airplane, have toincorporate at one and the same time antennae for transmission andreception in the VHF band, in particular from 100 to 156 MHz, and in theUHF band, in particular from 225 to 400 MHz. For reasons especially ofweight, it would be desirable to be able to use, for this application,one single transmitting and receiving antenna switchable between the twobands, VHF and UHF. The practical realisation of such a switchableantenna comes up against various problems, resulting more particularlyfrom the large bandwidth and the long range required for the antenna.

In particular, the realisation of a passive antenna that is switchablebetween the VHF and UHF bands is not practicable, because of the largebandwidth to be covered. Nor is it practicable to use, for theapplication under consideration, an antenna adapted with lossy elementsbecause such an antenna would offer too limited a range.

SUMMARY OF THE INVENTION

The switchable antenna for the VHF and UHF frequency bands according tothe present invention is characterised in that it comprises a capacitiveelement spaced from a reflecting surface constituting ground; a firstself-inductance, inserted between the capacitive element and a crossingpoint of the ground surface, connected to the transmitter/receiver andadaptable for VHF band use by short-circuiting of certain of itssections, and for UHF band use by short-circuiting of all its sections;a second self-inductance which a switch serves to connect between thefirst self-inductance and the earth surface for VHF band use and todisconnect for UHF band use; and also conductive side members insertedbetween the capacitive element and the ground surface on opposite sidesof the self-inductances.

When the antenna according to the present invention is switched to theVHF band, its aerial is essentially composed of the capacitive element,spaced from the reflecting surface. It accordingly presents a smallheight in relation to the VHF wavelength to which the aerial is adapted,due to the appropriate choice of electrical size of the firstself-inductance, by short-circuiting of certain of its sections. Theaerial then offers an adequate bandwidth. As the adaptiveself-inductances are inserted between the capacitive element and thereflecting surface, and are thus in some way a part of the aerialitself, they do not in practice cause any reduction in the bandwidth ofthe aerial, as would be the case if the two self-inductances weresignificantly spaced from the aerial, and connected to it by suitablecables. Furthermore, when the antenna according to the present inventionis switched to the UHF band, its aerial is of the classic type, referredto as having manchettes or side members, the radiating element or bladeconsisting of the capacitive element itself. The self-inductances arethen taken out of circuit, the first by short-circuiting of all itssections, and the second by disconnection. This very classic aerialoffers satisfactory standing wave ratio, particularly in the UHF bandfrom 225 to 400 MHz, without the need to use adaptive circuitsconsisting for example of self-inductances.

The present invention thus permits the realisation of a switchableantenna of the type indicated by means of a small number of simplecomponents, forming a whole that is compact, light and of smalldimensions, which is of particular advantage for application toaircraft.

The switches which are associated with the first self-inductance toshort-circuit its different sections, and the switch which is associatedwith the second self-inductance to connect or disconnect it can inprinciple be of any type. One could for example envisage using smallhigh-frequency relays. This solution would however present disadvantagesof space taken up, of poor reliability in a harsh environment, and ofover-long switching times. This is why, in a preferred form ofimplementation of the invention, the switches associated with the firstand second self-inductances incorporate semiconductor diodes, preferablyof the P-I-N type, which do not present any of the disadvantagesmentioned. Such diodes in fact have very short switching times, highreliability, even in harsh environments, and a very modest bulk, whichpermits them to be soldered directly to appropriate points of theself-inductances without preventing the complete aerial from beingcompact and of modest dimensions.

For preference, the switch associated with each section of the firstinductance incorporates at least one P-I-N diode, one electrode of whichis connected directly to a turn at one end of the corresponding section,and the other electrode of which is connected, on the one hand, to aturn at the other end of said section via a capacitor for shunting VHFor UHF currents and, on the other hand, to a source of continuousbiasing of the diode via a self-inductance for blocking VHF or UHFcurrents and a crossing point of the ground surface. When thepolarisation source applies to the diode an inverse biasing voltagewhich is high enough, e.g. -250 volts, it is blocked, and so the VHF orUHF currents flow in the corresponding section of the firstself-inductance. By contrast, when the dc bias power source supplies tothe diode a direct current of sufficient magnitude, e.g. 100 milliamps,the diode is rendered conductive, and the VHF or UHF currents areshunted away from the corresponding section of the firstself-inductance, via the shunt formed by the conducting diode, in serieswith the shunting capacitor, so that the said section of the firstself-inductance is short-circuited for the VHF or UHF currents.

In a particularly advantageous form of implementation of the antennaaccording to the present invention, its different components are fixed,or constituted by circuits printed on one single electrically insulatedboard which also carries the first self-inductance and the second one,and which can be surrounded by a radome of modest width, which is givenan aerodynamic profile. Such a combination can evidently be dimensionedin such a way as to present the modest weight and the modest bulkrequired for it to be fitted on an airplane, with the aerodynamicprofile of the radome, which embraces the antenna, conferring on thewhole a modest drag.

In this particularly advantageous embodiment, it would evidently bedesirable for the first and second self-inductances to consist ofcircuits printed on one and the same electrically insulating board, andfor the blocking self-inductances, associated respectively with thedifferent sections of the first self-inductance, to be positioned in theimmediate proximity of the corresponding diodes, the latter themselvesbeing positioned in the immediate proximity of the secondself-inductance. Although this embodiment of the antenna according tothe present invention appears to offer the greatest advantages as far ascompactness, low weight and low cost are concerned, experience andcalculations have shown that it presents the following disadvantage: ashas already been indicated above, when the antenna according to thepresent invention is switched to the VHF band, the height of its aerialis small in relation to the wavelength, so that the electricalequivalence circuit of the aerial contains a relatively small radiationresistance; in order to provide a sufficient power output from theantenna for the requisite range, it is thus necessary to feed its aerialwith high-intensity VHF currents, which cause a voltage surge to appearat the terminals of the first self-inductance. Consequently, those atleast of the blocking self-inductances which are associated with thesections of the first adaptive self-inductance which are furthestremoved from the reflecting surface are subject to voltage surges liableto produce stresses between their neighbouring turns. Furthermore, theelectrical values of these blocking self-inductances, intended toprevent the UHF and VHF currents from distorting the firstself-inductance through the medium of the biasing conductors and oftheir capacities in relation to ground, are influenced by the VHF or UHFcurrents circulating in the first self-inductance, to which the saidblocking self-inductances are adjacent. Finally, the VHF or UHFcurrents, being intense, would be passing via the first and secondself-inductances, printed directly on to the insulating board, produceexcessive heating of the first and second self-inductances.

This is why it is preferable for the adaptive self-inductances of theantenna according to the present invention not to consist of circuitsprinted on to a single electrically insulating board, but rather toconsist of corresponding electrical components independent of the saidboard, though they may be supported by it. In particular, the firstself-inductance is formed, for preference, by the helical winding ofsolid conductors, connected in such a way as to continuously bias thediodes associated with at least some of the sections of the firstself-inductance, and of at least one hollow conductor, such as ametallic sheath, surrounding the solid conductors without contact andconnected in such a way as to conduct only VHF and UHF currents.

With this embodiment, it is possible to position the blockingself-inductances at an appreciable distance from the first adaptiveself-inductance, so that the said blocking self-inductances are notinfluenced by the VHF and UHF currents flowing in the firstself-inductance; this assumes, of course, that the bias voltages aretransmitted to the diodes associated with the differrent sections of thefirst self-inductance by conductors of appropriate lengths, preferablysolid conductors; but, as the latter are surrounded by the metallicsheath, which conveys the VHF and UHF currents, they are screened fromthe influence of the latter and consequently from the over-voltages thatthey produce along the turns of the first self-inductance. Finally,however intense the VHF or UHF currents circulating in the metallicsheath wound in a spiral may be, they cannot induce in it excessiveheating.

In practice, the first self-inductance of the antenna according to thepresent invention can be formed, for example, by the helical winding ofcoaxial cables, soldered by their sheaths and having lengths justsufficient to enable their respective central conductors to bias thediodes associated with at least some of its sections. In the case inwhich each diode associated with a section of the first self-inductancehas an electrode connected directly to the end turn of the section whichis furthest away from the reflecting surface, the longest coaxial cablecan be replaced by one single simple conductor, either solid or hollow.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, a description is given below, and illustrated inoutline in the accompanying drawings, of an embodiment of the antennaaccording to the present invention. In the drawings:

FIGS. 1 and 2 are, respectively, a view from the side and a view fromthe front showing this embodiment, much simplified and reduced to itsmain components.

FIG. 3 is the electrical equivalence circuit diagram of the antenna ofFIGS. 1 and 2.

FIG. 4 is an electrical circuit diagram of an embodiment of a diodeswitch, associated with one of the sections of the first self-inductanceof the antenna according to the present invention.

FIG. 5 is a side view corresponding to FIG. 1 and showing the whole ofthe components of the antenna according to the present invention.

FIGS. 6, 7 and 8 are front, side and plan views of the whole of theantenna of FIG. 5 and of its radome, in an embodiment capable of beingmounted on the nose of a military jet airplane.

DETAILED DESCRIPTION OF THE DRAWINGS

In the outline FIGS. 1 and 2, 1 designates the ground of the antenna,which constitutes a reflecting surface for its aerial, and which isformed for example by its metallic base, as will be indicated in moredetail below. 2 designates a capacitive element, for example a thinplate, perceptibly rectangular, made of copper, positioned at anappropriate distance from the reflecting surface 1; 3a and 3b designatetwo thin metal plates which are inserted between the reflecting surface1, with which they respectively make contact along their correspondingedges, and the capacitive element 2, with which they do not makecontact. 4 designates a first adaptive self-inductance, which isinserted electrically between the capacitive element 2 and a crossingpoint 6 of the ground surface 1, connectable, more particularly by acoaxial cable, to the output of a transmitter/receiver capable of beingswitched to the VHF frequency band, in particular between 100 and 150MHz, and to the UHF frequency band, in particular between 225 and 400MHz.

Of course, the crossing point 6 could equally well be connected, bymeans of coaxial cables and a tee-piece union, to the respective outputsof a VHF transmitter/receiver and another, UHF transmitter/receiver. Thefirst self-inductance 4 incorporates, in the embodiment illustrated inFIG. 1, three sections each of which can be shunted by one of theswitches 7a, 7b, 7c. Another switch, 8, allows one of the terminals of asecond adaptive self-inductance 5, whose other terminal is connected tothe ground 1, to be connected to the crossing point 6. As can be seen inFIG. 1, the two self-inductances 4 and 5, as well as the switches 7a to7c and 8 which are associated with them are disposed in the spacebetween the components 1, 2, 3a and 3b, so that the antenna as a wholehas a form that is compact and, as can be seen in FIG. 2, of modestwidth, and so that it can be inserted into a radome 9, all of whosedimensions, but above all its width (FIG. 2), are small.

In FIG. 3, which represents the electrical equivalence circuit diagramof the antenna of FIGS. 1 and 2, the components corresponding to thoseof FIGS. 1 and 2 have been marked with the same reference numerals: Cadesignates the capacitance of the capacitative element 2 (FIGS. 1 and 2)in relation to the ground surface 1, and Ra designates the radiationresistance of the aerial.

The antenna according to the present invention, which has just beendescribed with the help of FIGS. 1 and 2, functions in the followingmanner:

The switches 7a, 7b, 7c and 8 are controlled by known means, which havenot been depicted in FIGS. 1 and 2 and which it is not necessary todescribe in detail. An examplary embodiment of these means will bedescribed below. The switching of the antenna of FIGS. 1 and 2 to theVHF band is achieved by closing the switch 8, which connects the secondself-inductance 5 in parallel with the crossing point 6; the tuning ofthe antenna to the frequency of the VHF signals that it receives isobtained by switching the first self-inductance 4 to an appropriatevalue; this first self-inductance 4, in the case in which it is composedof three identical sections, that is to say each incorporating the samenumber of turns, can take on a maximum value when the three switches 7ato 7c are open, a minimum value when one only is open, and anintermediate value when two of the said switches are open. In general,the first self-inductance 4 incorporates a much larger number ofsections, which may not be identical among themselves, and whose diversecombinations, corresponding to the diverse possible configurations ofthe switches associated with the said sections, allows a number ofdifferent values, much greater than three, to be given to theself-inductance 4. Of course, the different values that can thus beassumed by the first self-inductance 4, in series with the radiationresistance Ra, and the single value of the second self-inductance 5, inparallel with Ra, are chosen in such a way as to compensate for thereactance of the capacitance Ca, so as to minimise the standing waveratio of the aerial, preferably making it lower than 2. In the VHF band,the active components are thus only the components 2, 4 and 5, theaerial consisting essentially of the capacitive element 2, separatedfrom the reflecting surface 1 in such a way as to form a monopole ofsmall height in relation to the wavelength, to which there corresponds(according to FIG. 3) a capacitative impedance having a fairly smallresistive term, Ra. The large bandwidth obtained by this VHF aerialresults in particular from the fact that the adaptive self-inductances,4 and 5, are located in the immediate proximity of the other components,1 and 2.

The functioning of the antenna of FIGS. 1 and 2 in the UHF band isobtained, when all the switches 7a to 7c are closed, so as toshort-circuit all the turns of the first self-inductance 4, and, whenthe switch 8 is open, in such a way as to disconnect the secondself-inductance 5. The only active components of the aerial are then thecomponents 1, 2, 3a and 3b, the latter two constituting "manchettes",utilised classically in UHF antenna of this type, known as "sabreantennae". The UHF antenna obtained in this way has appropriateimpedance for presenting a low standing wave ratio in the whole of theUHF band, for example from 225 to 400 MHz, that is to say it is possibleto utilise this UHF antenna in a wide band of frequencies without itsneeding to be associated with switchable adaptive components such asself-inductances 4 and 5.

FIG. 4 is the electrical circuit diagram of an embodiment of one of theswitches, for example 7b, which is associated with one of the sections,in particular 4b, of the first self-inductance 4. In this form ofimplementation, the switch 7b incorporates a semiconductor diode, 10b,preferably of the P-I-N type, one of whose electrodes, in particular thecathode, is connected directly to the end turn, 4b1, of section 4b,while its other electrode, in particular its anode, is connected, on theone hand, to the other end turn, 4b2, of section 4b, via a capacitor11b, whose capacitance is chosen such that it produces a weak reactancefor VHF and UHF currents, so that the latter are shunted via thiscapacitor 11b and the diode 10b when the latter is conductive which hasthe effect of deactivating section 4b of the first self-inductance 4:the anode of the diode 10b is on the other hand connected to a source ofcontinuous biasing, via a self-inductance, 12b, whose value is chosensuch that it produces a very high reactance in the VHF and UHF bands, soas to avoid shunting of the VHF or UHF currents, flowing in the shunt10b-11b when the diode 10b is conductive at least partially towards thebiasing source; the latter, which is not shown in FIG. 4, is connectedby suitable means, which are known, and likewise are not shown, to theend of a conductor 13b, which is connected in series with the blockingself-inductance 12b, and which crosses the ground surface 1 at acrossing point 14b of specified capacitance. The self-inductance 15,which is inserted between the earth surface 1 and one of the turns ofthe first self-inductance 4, serves to return to ground the polarisationcurrent which has passed through the diode 10b to render it conductive,while still avoiding, due to its high reactance, shunting to ground ofthe VHF or UHF currents which flow in the first self-inductance 4. Ifthe diode 10b is for example of the DH438-08 type, it can be renderedconductive by supplying to the conductor 13b for example a directcurrent of 100 milliamps, and it can be blocked by applying to the sameconductor for example an inverse voltage of -250 V.

FIG. 5 represents in outline a form of implementation of the antennaaccording to the present invention, in which the first self-inductance 4comprises five sections, 4a to 4e, with each of which is associated aP-I-N diode switch of the type illustrated in FIG. 4. It is thusunnecessary to describe again the composition of each of these switches:suffice it to say that the switches associated with the two sections ofthe self-inductance 4, those nearest to the ground surface 1, that is tosay sections 4a and 4b, incorporate switches which are each providedwith two P-I-N diodes, for example 10a1 and 10a2, connected in parallelwith one another, and preferably identical among themselves, so that thebiasing current is divided approximately equally between them: thisarrangement has the advantage of limiting the thermal power dissipatedat the level of one junction of each P-I-N diode.

In the preferred form of implementation, which is illustrated in FIG. 5,the first self-inductance, 4, which comprises five sections, 4a to 4e,is formed by the helical winding of four coaxial cables, soldered bytheir sheaths, and by a simple conductor, solid or hollow, whoseexternal diameter is preferably close to that of the coaxial cables, towhose sheaths it is likewise soldered. The four coaxial cables, thefirst ends of the metallic sheaths of which have been designated 15b1 to15e1 respectively, as well as the simple conductor, are of differentlengths, for example in arithmetical progression so that the foursections 4a to 4e of the self-inductance 4 each incorporate the samenumber of turns, of the same diameter, so that each section can be seento correspond to one-fifth of the value of the total self-inductance. Inthese conditions, section 4a, the one nearest to the ground surface 1,is formed by helical winding in juxtaposition of the four coaxial cablesand the simple conductor, section 4b is formed merely by helical windingof the three end coaxial cables 15c1 to 15e1 and of the simple conductor. . . etc., section 4d being formed by helical winding of the single endcoaxial cable 15e1 and of the simple conductor, which in itself aloneconstitutes the fifth section, 4e. The second ends of the sheaths of thefour coaxial cables which in practice scarcely emerge from the helicalwindings have been designated 15b2 to 15e2, while in FIG. 5 these endshave been depicted in a very elongated form, to make the figure morelegible; the second end, for example 16b2, of the central conductor ofeach coaxial cable, for example of the one whose second sheath end isdesignated 15b2, is connected to the common point of the anode of thediodes 10b1 and 10b2 and of the VHF and UHF current shunt condenser,11b, of the switch associated with the immediately following section ofthe self-inductance 4, for example its section 4b. In the same way, thesecond end, 16e2, of the central conductor of the coaxial cable whosesecond sheath end is designated 15e2 is connected directly to the commonpoint of the single diode 10e and of the shunt condenser 11e of theswitch associated with section 4e which is formed exclusively by windingof the end part of the simple conductor. In practice, each cathode ofthe diode or diodes associated with one of the five sections, as well asone of the plates of the corresponding shunt capacitor, is solderedrespectively to the corresponding ends of the coaxial cables, as near aspossible to the helical windings constituting the self-inductance 4.

On the other hand, the first ends, 16b1 to 16e1, of the centralconductors of the four coaxial cables whose first sheath ends aredesignated 15b1 to 15e1, are connected, in series respectively withblocking self-inductances 12b to 12e, to conductors 13b to 13e whichtraverse the ground surface 1 via crossing points 14b to 14e, beyondwhich the ends of the said conductors 13b to 13e can be connectedrespectively to the outlets of a biasing device, which will be describedin greater detail below, and which is capable of applying suitablecontinuous bias voltages to the said conductors 13b to 13e. Thesecontinuous bias voltages, which may for example have the valuesindicated previously, are transmitted, via the blocking self-inductances12b to 12e, by the central conductors of the four coaxial cables, to theanodes of the diodes of the switches associated respectively withsections 4b to 4e of self-inductance 4. The anodes of the pair ofdiodes, 10a1 and 10a2 of the switch associated with section 4a, percontra, receive their continuous bias voltages, via the blockingself-inductance 12a, directly by way of a simple conductor 17a.

Due to the arrangement which has just been described, the VHF or UHFcurrents which flow in the sheaths of the coaxial cables have noinfluence on their central conductors, where the biasing currents flow;consequently, the VHF or UHF voltage applied to the second end of eachof the central conductors is perceptibly the same as that applied to itsfirst end, since the high frequency voltages applied respectivelybetween 15b2 and 16b2, between 15c2 and 16c2, between 15d2 and 16d2, andbetween 15e2 and 16e2 are in practice nil: thus one avoids thesubjection of the blocking self-inductances 12b, 12c, 12d and 12e tovery high VHF or UHF over-voltages, which would be liable to damage themor to disturb their functioning. On the other hand, the P-I-N diodes10b1, 10b2, 10c and 10d bear over-voltages when they are blocked.Furthermore, the VHF or UHF currents which flow in the sheaths of thefour coaxial cables do not give rise to any heating harmful to thelatter, as would be the case if they flowed in windings printed on aninsulating board.

The electrical circuit of the antenna illustrated in FIG. 5 incorporatesthe following components besides: the second adaptive self-inductance,5, which can consist of a single coaxial cable or of a conductivewinding, printed on an insulating board, coupled at one end directly tothe ground surface 1. Its other end is connected to the cathode of adiode 18, for example of the P-I-N type, whose anode can receivecontinuous bias voltages, via a blocking self-inductance 19, by way of aconductor 20 which traverses the ground surface 1 at a crossing point21. The anode of the diode 18 is connected to a common electrical point,which can be formed by a conductive strip, made for example of copper,22, and to which is connected, via a capacitor 23, presenting a weakimpedance for VHF and UHF currents, a conductor 24 which traverses theground surface 1 via the crossing point 6 and which can be coupled tothe outlet or outlets of one or more VHF and UHF transmitter/receivers,by means not shown, in particular coaxial cables. Finally, a capacitor26, whose reactance is selected to provide a precise match to the UHFband, is inserted between the common electrical point 22 and the end ofthe first self-inductance 4 nearest to the ground surface 1, that is tosay the ends, 15b1 to 15e1, of the sheaths, soldered among themselves,of the four coaxial cables. In parallel with this capacitor 26 there ismounted a diode switch which allows it to be short-circuited in the VHFband; in the form of implementation illustrated, this switch consistsessentially of a pair of diodes, 271 and 272, for example of P-I-N typewhose cathodes are connected to the end of section 4a of theself-inductance 4, the nearest to the ground surface 1, while theiranodes are connected in parallel to the common electrical point 22, viaa capacitor 28, presenting a weak reactance for VHF currents. Aconductor 29, traversing the ground surface 1 via a crossing point 30,allows suitable continuous bias voltages to be applied to the anodes ofthe diodes 271 and 272 via a blocking self-inductance 31.

The antenna illustrated in FIG. 5 and described above functions in thefollowing manner: for operation in the VHF band, a suitable continuousbias voltage, more particularly a direct current of suitable intensity,is transmitted via the conductor 29 to the diodes 271 and 272 in such away as to render them conductive and to short-circuit the capacitor 26.The adaptation of the value of the first self-inductance 4 to the VHFfrequency, of transmission or of reception, which has been selectedresults from the application of direct bias currents to those of theconductors 13a to 13e which correspond to those of the sections 4a to 4ethat have to be short-circuited by the corresponding diodes and shuntcapacitor, while inverse blocking voltages are applied to the otherdiodes, by means of the corresponding conductors. As has already beenshown, the direct bias currents return to the ground by means of thesheaths of the coaxial cables, to which the cathodes of the diodesmentioned are connected, as well as via the blocking self-inductance 15.Finally, a suitable direct current is supplied, by means of theconductor 20, to the diode 18, so as to render it conductive and thus toinsert the second self-inductance 5 into the adaptive circuit, via thecommon electrical point 22.

For operation in the UHF band, on the other hand, direct bias currentsare supplied to all the conductors 13a to 13e to render conductive thediodes of the switches associated with all the sections, 4a to 4e, ofthe first self-inductance, 4, which is thus totally short-circuited. Aninverse blocking voltage is applied to the diodes 271 and 272 by theconductor 29, so that the capacitor 26 is not short-circuited.Similarly, a blocking voltage is applied by the conductor 20 to thediode 18, which thus isolates the second self-inductance 5 from the restof the circuit.

As has already been shown, the components 2, 3a, 3b and 5 are composedpreferably of metallic deposits, more particularly copper deposits, onan electrically insulating board, for example a synthetic resin loadedwith glass fibres; the other components, 10a1 to 10e, 11a to 11e, 12a to12e, 18, 19, 23, 26, 271, 272 and 28, like the four coaxial cables andthe simple conductor, constituting the first self-inductance 4, canequally be carried by the same insulating board, being arranged inrelation to elements 1, 2, 3a and 3b, preferably as illustrated in FIG.5.

FIGS. 6 to 8 show the external appearance of an antenna according to thepresent invention, intended in particular to be fixed below the nose ofa military jet airplane. 32 designates a metal plate below which thereis fixed the radome 9, whose aerodynamic profile can be seen well,especially in the plan view of FIG. 8. The edges of the metal plate 32are perforated by holes, such as 32a, for bolts to pass through to fixthe said plate to the skin of the aircraft. The plate 32 is joined tothe body of the aircraft in such a way as to form the reflecting surfaceof the aerial, designated 1 in the figures previously described. FIG. 6shows in dotted lines the section of the electrically insulating boardon which all the components of the antenna, previously described, areprinted or fastened. On the surface of the plate 32 opposite the radome9 a metal casing 34 is fixed, which is thus mounted underneath the skinof the aircraft; the lower surface (in FIGS. 6 to 8) of the casing 34consists of the metal plate 32, on which the insulating board 33 and theradome 9 are fixed on edge. On the front face of the casing 34, whichcan be seen in FIG. 6, are fixed a coaxial connector 35 which isconnectable by a coaxial cable--not shown--to the coaxial output(s) orinput(s) of one or more VHF and/or UHF transmitter/receivers, as well asa multipin connector 36.

Inside the casing 34 different devices are mounted of which one form ofembodiment will be indicated by way of non-restrictive example: thisinvolves first of all a decoder of signals indicating the tuningfrequency of the antenna and originating for example from thetransmitter/receiver, by way of certain pins of the connector 36, forexample in the known format referred to as "ARINC Series". This decoderproduces switching signals whose use will be shown a little further on.The casing 34 also contains a converter for the electrical supplycurrent which it receives by way of other pins of the connector 36,originating for example from the on-board generator of the aircraft, at28 V D.C. This converter produces, for example on two distinctterminals, a current which can rise to 2 amps at a voltage of +5 V, anda current which can rise to 150 microamps at a voltage of -250 V. Thecasing 34 finally contains a selector, generally electronic, which canbe a circuit of a known type, which need not be described; this selectoris linked to two output terminals of the converter of the currentsupply, and receives also the switching signals produced by the decoder;it is set up in such a way as to apply to at least certain of the lines13a to 13e, 20 and 29 (FIG. 5) direct currents of for example 100milliamps or inverse voltages of for example -250 V, in terms of theswitching signals which it receives from the decoder.

The present invention is not limited to the embodiments described above:it embraces all of their variants, of which a few only will be indicatedbelow, by way of non-restrictive example:

The casing 34 and the circuits that it contains are susceptible tonumerous different embodiments. The form and the arrangement of the base32 and of the radome 9 are matters for choice. In the case of aterrestial antenna, or one intended for vehicles, whose weight and bulkrequirements are less strict, the different components could bedistributed on several insulating boards, or could even all be composedof discrete components, including the components 2, 3a and 3b, whichcould then be copper plates of greater or lesser thickness. Instead ofcomprising identical sections, the first self-inductance 4 couldcomprise sections which differ from one another in such a way as to beswitchable to values forming for example a binary progression. In thecase of the embodiment illustrated in FIG. 5, the longest simpleconductor, intended to form by itself the section 4e, could be replacedby a fifth coaxial cable, whose sheath would then be connected to thecathode of the diode 10e, and its central conductor to its anode, theends 16e2 to 16b2 of the centrral conductors of the four other coaxialcables then having to be connected respectively to the anodes of thediodes 10d, 10c, 10b1-10b2, and 10a1-10a2; in this case, of course, theblocking inductance 12a would have to be connected to the other end ofthe central conductor of the extra coaxial cable. The continuous biassigns could be inverted, given corresponding inversions of the diodes10a1 to 10e, 18, 271 and 272. The first self-inductance 4 could also beconstituted by the helical winding of a single coaxial cable, presentingthe following characteristic structure: it would comprise solidconductors equal in number to the sections of the self-inductance 4 anda single metal sheath, surrounding without contact all these solidconductors, which could for example be insulated from it by solidinsulation. Of course, each of the solid conductors would have totraverse the sheath via an insulating crossing point, to proceed toapply the continuous polarisation to the diode of the switch associatedwith one of the two nearest sections. As has already been indicated,each of the switches associated with one of the sections of the firstself-inductance 4 could incorporate, instead of one or two P-I-N diodes,another switching component, adapted to VHF and UHF frequencies, whetherit be a solid state component or a component of anther type, for exampleelectromagnetic.

What is claimed is:
 1. An antenna switchable between VHF and UHFfrequency bands, comprising:a reflecting surface constituting a ground;a capacitative element spaced from said reflecting surface; a firstinsulated conductor passing through said reflecting surface andconnectable to a transmitter receiver adapted to be switched between VHFand UHF frequencies;a first self-inductance comprising at least in partby at least one hollow, helically wound conductor and electricallyinserted between said capacitive element and said first insulatedconductor, and including several sections; a second insulated conductorpassing through said reflecting surface; a plurality of first switches,each corresponding to one of said several sections for short-circuitinga respective section, and comprising at least one P-I-N diode having afirst electrode connected to a first end point of said correspondingsection and a second electrode connected to a second end point of saidcorresponding section through a capacitor having low resistance for VHFand UHF frequencies, and also connected to said second insulatedconductor; means for continuously biasing said diode via aself-inductance connected to a conductor which is helically wound andsurrounded without contact by said hollow conductor and connected tosaid second insulated conductor; a second self-inductance connectedbetween said first self-inductance and said reflecting surface; a secondswitch connected to said second self-inductance and adapated to bedisconnected from said first self-inductance; and conductive sidemembers inserted between said capacitive element and said reflectingsurface on opposite sides of said first and second self-inductances andconnected to said reflecting surface.
 2. An antenna according to claim1, wherein a capacitor to adapt the antenna for UHF band use is insertedbetween the corresponding ends of said first and secondself-inductances, and a switch comprising at least one P-I-N diode isconnected in parallell with said capacitor.
 3. An antenna according toclaim 1, wherein a single electrically insulated circuit board carriessaid first and second self-inductances, and is surrounded by a radomewhich has an aerodynamic profile.
 4. An antenna according to claim 3,wherein circuit components of the antenna are printed on said circuitboard.
 5. An antenna according to claim 3, wherein said circuit boardand the radome have their bases fixed on edge by a metal plate whichforms a part of said reflecting surface, and a casing is fixed on saidmetal plate opposite said circuit board and radome.
 6. An antennaaccording to claim 1, wherein said first self-inductance partly consistsof sheaths of coaxial cables, said sheaths being electrically connectedwith one another, and said conductor is the inner wire of said coaxialcables corresponding to the preceding section of said self-inductance inthe direction of said capacitive element.
 7. An antenna according toclaim 1 wherein said first self-inductance consists of a plurality ofcoaxial cables of different length and a simple cable longer than any ofsaid coaxial cables, the sheaths of said coaxial cables and said simplecable being electrically connected together with all said cables havingone end at the same point, and the inner wires of said coaxial cablesare connected by one end to said second electrodes of said P-I-N diodesand by the opposite end to said self-inductance for blocking VHF and UHFsignals, and the end of said sections of said first self-inductance arethe ends of the sheath of said at least one of said coaxial cable andsaid simple cable.
 8. An antenna according to claidm 1 wherein saidsections of said first self-inductance are identical in length.
 9. Anantenna according to claim 1 wherein said sections of said firstself-inductance are different from one another in length.